About aniseikonia

Definition of aniseikonia

Translated from Greek aniseikonia means "unequal
images". It is a binocular condition, so the image in one eye
is perceived as different in size compared to the image in the other eye. Two different types of aniseikonia can be differentiated: static and
dynamic aniseikonia.

Static aniseikonia or aniseikonia
in short means that in a static situation where the eyes are
gazing in a certain direction, the perceived (peripheral) images are
different in size (see Fig.1).

Dynamic aniseikonia or (optically
induced)
anisophoria means that the eyes have to rotate a
different amount to gaze (i.e. look with the sharpest vision) at the same
point in space (see Fig. 1). This is especially difficult for eye
rotations in the vertical direction.

In the remainder of this web-page we will speak of
aniseikonia and anisophoria instead of static aniseikonia and dynamic
aniseikonia. Aniseikonia will be the target parameter to be measured
and corrected. By contrast, anisophoria can more or less
been determined from the prescription and is a parameter that needs to be
kept within bounds when correcting the aniseikonia.

Aniseikonia values by
definition represent how much the right eye should be magnified or
minified to cancel the aniseikonia. For example, a measured
aniseikonia of -5% means that the image is in the right eye is perceived
as approximately 5% larger than the image in the left eye and that
therefore the aniseikonia is corrected by minifying the image in the right
eye by 5% (or magnifying the image in the left eye by approximately 5%, or a combination of
both).

With optically-induced aniseikonia (i.e.
aniseikonia due to a difference in the optical magnification properties of
the eyes + auxiliary optics), the aniseikonia is sometimes described
by means of the aniseikonic ellipse. This ellipse
describes the aniseikonia in all directions. It consists of an overall
aniseikonia and a meridional aniseikonia. The
overall part describes the minimum (closest to zero) aniseikonia and the
meridional part describes the direction-dependent part of the aniseikonia
(usually induced by the cylinder part of the prescription).

In the case of retinally-induced aniseikonia, the aniseikonia
most likely varies with field angles (we call
this field-dependent aniseikonia). The field angle is
defined as the angle between the gaze direction and the direction of a
peripheral point (see Fig. 2). With field-dependent aniseikonia, one
cannot speak of the aniseikonia anymore, since the aniseikonia varies
depending on from which location on the retina the aniseikonia is determined. For
more information on field-dependent aniseikonia, see our article:
Retinally-induced
aniseikonia1.

Patients at risk of aniseikonia

Fig. 2 schematically shows how an eye perceives
the size of an image. First objects in the outside world are imaged
with a certain optical magnification (minification) on the retina.
Next, the retinal receptors sample the retinal image into 'pixels'.
Finally, this information is processed by the brain.

Fig. 2: Schematic presentation of the different steps to get to
a perceived
image size and the visualization of a field angle a..

Aniseikonia could basically arise if
there is a difference between the eyes in any of the three steps depicted
in Fig. 2. Optically-induced aniseikonia patients
might be anisometropes, pseudophakes,
and refractive surgery patients. Retinally-induced
aniseikonia patients are those patients in which the retina may be
compressed or stretched due to an ocular condition or surgery. Due
to the compression or stretching, an image projected onto the retina will
be sampled by either a greater pr lesser number of receptors, causing the perceived image to appear
bigger or smaller (macropsia or micropsia). There may also be a more
random (local) distortion, called metamorphopsia. Retinally-induced
aniseikonia patients are for example patients with an epiretinal
membrane (macular pucker), a retinal detachment, a macular
hole, macular edema, or a retinoschisis.
We have not found any cortically-induced aniseikonia
patients in the literature.

Aniseikonia symptoms

Bannon and Triller2 reported a list of characteristic aniseikonia
symptoms based on a study with 500 patients (see Table 1). Burian3
published a similar list presenting the symptoms the patients most
frequently complained about. His main categories were I. Astenopic
symptoms, II. Impaired fusion and/or poor stereopsis, and III. Anomalous
spatial localization.

Table 1:
Characteristic symptoms of aniseikonia patients

Symptom

Percentage of patients

Headaches

67%

Astenopia (fatigue, burning,
tearing,
ache, pain, pulling, etc)

67%

Photophobia

27%

Reading difficulty

23%

Nausea

15%

Motility (diplopia)

11%

Nervousness

11%

Vertigo and dizziness

7%

General fatigue

7%

Distorted space perception

6%

Clinically significant aniseikonia values

Aniseikonia seems to become
clinically significant at values of 3-5%4-8.
Sometimes sensitive individuals are suspected to have symptoms with less
aniseikonia9, but it is well possible that these symptoms are
caused by optically-induced anisophoria and not aniseikonia.
The topic of clinically significant aniseikonia values warrants further research. It is unclear what the
contribution to aniseikonic symptoms is of aniseikonia versus anisophoria. In case of (retinally-induced) field-dependent
aniseikonia, it is also unclear which field angles and field directions are
causing the most symptoms.

Testing of aniseikonia

In older optometric/ophthalmic
textbooks, rules of thumb have been defined to correct aniseikonia,
without actually testing for the amount of aniseikonia. These rules
are based on Knapp's law, which deals with an image size difference as
projected onto the retina in anisometropia
(i.e., only optical effects are taken into account). Eye care
professionals using these rules will base treatment on the patient's
prescription and perhaps the difference in corneal curvature or eye
length between the eyes. However, in the more recent literature it
has been well established that even in anisometropia, the retinal receptor
distribution may also play a role,10-14 and
therefore these rules of thumb should not be used. Instead the
aniseikonia should be measured.

There are basically two methods to test for aniseikonia:
the space eikonometric method and the
direct comparison method.15,16 The space eikonometric method is based on binocular
space perception, while the direct comparison method is based on directly comparing
perceived image sizes between the two eyes. Table 2 compares the
space eikonometer with the three commercially available direct comparison tests: the New Aniseikonia
Test (Awaya test), the Basic Aniseikonia Test (version 1), and the Aniseikonia Inspector (version
3).

Because it is the most complete product for aniseikonia management, we
will now discuss the aniseikonia test of the Aniseikonia Inspector,
version 3. The patient looks through red-green glasses at the
computer screen, which gives a series of test images (see Fig. 3a for one
such an image). For each test image the patient's task is to
identify which of the two I-shaped bars is perceived as larger (a forced
choice procedure). Note that because of the red-green glasses, one eye
sees only the left I-bar and the other eye sees only the right
I-bar. If all images of a series have been presented, the data can
be plotted in a 'psychometric curve' (see Fig. 4). Using a maximum
likelihood mathematical method, the aniseikonia value can be obtained from this
data. One of the advantages of this forced choice method compared to a
method of adjustment (such as used in the Basic Aniseikonia test and the 'New' Aniseikonia
Test) is that the forced choice method also provides information about the
consistency/accuracy of the measurement. This is also very useful
when the patient self-administers the test and the results are analyzed
elsewhere.

a
b

Figure 3: a) Example of a single aniseikonia test
image, b) same aniseikonia test image as on the left, but now with an (exaggerated)
vertical fixation disparity compensation (click on image to enlarge).

Figure 4: Raw aniseikonia test data. Each
data point represents the result of one presentation as for example shown
in Fig. 3, each with different I-bar sizes. The transition where the
left I-bar is perceived as larger to where the right I-bar is perceived as
larger is the patient's aniseikonia.

If a fixation disparity
is present (which might occur especially in the horizontal direction), the
aniseikonia test is more difficult to do because the two I-bars will not
be symmetrically aligned next to each other. In software it is easy
to compensate for a fixation disparity by shifting the I-bars relative to
each other (see for example Fig 3b). So when a patient with a
certain vertical fixation disparity looks at Fig. 3b through the red-green
glasses, he will see the image as Fig. 3a with the I-bars symmetrically
next to each other.

The
aniseikonia testing discussed above has all been 'objective'
testing. However, an eye care provider serious about aniseikonia
management should also have a set of
size lenses for
subjective testing. Size lenses are lenses without refractive power
(the image stays sharp), but with an optical magnification. They may
provide the patient with a sense of what it would be like to have a
certain aniseikonia correction.

How to use size lenses

If an aniseikonia test showed that a patient has 4%
aniseikonia, this means that the image in the
right eye (OD) needs to be magnified by 4% to cancel the aniseikonia.

To see what the effect of a 4% aniseikonia correction is for the
patient, a 4% size lens can be held in front of the right eye with
the concave side towards the eye (see top-left image).
Note that aniseikonia is a binocular condition and therefore a
relative measure between the two eyes. Instead of
magnifying the image in the right eye, equal image sizes are also
obtained by minifying the image in the left eye (OS). A size
lens minifies by flipping it as in the top-right image.
The lower two images give two more alternative ways to correct a
4% aniseikonia. They also show that with two size lenses, it is
possible to correct a larger amount of aniseikonia. Using both
the 7% and the 6% allows you to correct appr. 13%!

Correcting aniseikonia

Sometimes a patient suffering from aniseikonia is told that his/her problem can be solved by covering (fully
occluding) one eye. Of course, no patient is happy about that
'solution', and it should only be a last resort. Aniseikonia and its accompanying aniseikonic symptoms can
often be reduced
by changing the magnification properties of the auxiliary optics of the
patient. There are basically 4 possible optical solutions, each with
its own advantages and disadvantages. Depending on the patient's
case (optically or retinally induced aniseikonia, refractive errors of the
eyes, amount of aniseikonia to be corrected, whether contact lenses are an
option, importance of cosmetics, etc.) the best solution will be different
from patient to patient and might be a combination of the different
solutions.

Below 4 possible optical solutions
will be discussed assuming the amount of aniseikonia has been measured.
Even though it is best practice to measure the aniseikonia before
designing and prescribing a specialized correction (see section on
testing aniseikonia), not all eye care providers
may have testing equipment. In case of optically-induced aniseikonia
such as associated with anisometropia, the eye care provider could first
examine if contact lenses would improve the vision comfort enough.
Contact lenses fully eliminate any optically induced anisophoria
(dynamic aniseikonia) and often lessen the (static) aniseikonia.
Alternatively to contact lenses, the ShawTM spectacle lenses
could be tried. These are specifically aimed at reducing the optically
induced anisophoria and may also reduce the aniseikonia. How much and if
the aniseikonia will be reduced can only be known though by measuring
the aniseikonia first.

Optical solution 1 (contact lenses only)If
a patient tolerates the use of contact lenses, the first option that
should be investigated (calculated after the measurement of aniseikonia) is what the remaining aniseikonia
would be if all of the patient's prescription were put into contact
lenses. Especially with anisometropia
it has been shown that contact lenses often (but not always) give less
aniseikonia than regular spectacle lenses. If this solution happens
to work, then there only seems to be advantages. This
solution does not compromise the visual acuity, it is cosmetically very
acceptable, there is no anisophoria (contact lenses move with eye
rotations, so do not induce any prism effects with eye rotations), and it is relatively low cost.
To get a subjective experience of how much this solution would
improve the vision comfort, the patient should look first through some
trial contact lenses.

Optical solution 2 (glasses only, good VA)This
is the classic aniseikonia correction. Optical magnification changes
are induced by changing the shape of the spectacle lenses (curvature,
thickness) and/or the distance of the lenses to the eyes. This
solution does not compromise the central visual acuity. Because of the
changes in shape of the spectacle lenses, the cosmetics of the glasses
might be compromised (depending on the amount of aniseikonia to be
corrected). In optically-induced aniseikonia both aniseikonia and anisophoria
are often reduced at the same time. In retinally-induced
aniseikonia a correction of the aniseikonia may often result in the
introduction of anisophoria, so a balance needs to be found between how
much aniseikonia is corrected and how much anisophoria is
introduced. The amount of introduced anisophoria for this solution
may be less though than that of solutions 3 and 4. The cost of these spectacle lenses might be a
little higher than standard spectacle lenses, because of the non-standard
thickness and curvature of the lenses. To get a subjective
experience of how much this solution would improve the vision comfort,
the patient should look through his/her habitual glasses and a
size
lens.

Optical solution 3 (glasses only,
compromised VA)In
this solution, the optical magnification change is accomplished by altering the
refractive power of one of the spectacle lenses. As a consequence
the image is blurred in one eye, reducing the effective visual acuity
in that eye. Since the other eye is supposed to have a good visual
acuity, the overall binocular visual acuity should not be affected too
much. However, the blurring may cause visual discomfort (for
example, if the blurred eye is strongly dominant). Also, the patient's eye
care professional will need to determine if a decoupling of the central
vision due to the blurring in one eye is likely to cause, for example,
eye alignment problems. If the refractive power change
results in an overcorrection (some plus-power added), then the glasses may
become useful as monovision glasses (one eye sees sharp at far, and one
eye sees sharp at near). The spectacle lenses used for this solution
can be standard spectacle lenses, so the cosmetics as well as the cost
are favorable compared to solution 2. The resulting amount of anisophoria
will depend mainly on the final refractive power
difference between the two spectacle lenses (for retinally-induced
aniseikonia, this will often mean that the amount of anisophoria will increase). The
anisophoria may be less or more than
that of solution 4, depending on the refractive state of the eyes and the
aniseikonic correction necessary. To get a subjective experience
of how much (and if) this solution would approximately improve the vision
comfort, the patient could be fitted with a trial frame with standard
trial lenses.

Optical solution 4 (contact - glasses
combination)This
solution is similar to solution 3, but now one contact lens is added to
restore the compromised visual acuity. The cosmetics
will be good, as standard spectacle lenses can be used. The cost
will go up some, because now both glasses and a contact lens are used at
the same time. The anisophoria may be less or more
than that of solution 3, depending on the refractive state of the eyes and
the aniseikonic correction necessary. For an isometrope (the two
eyes appr. equal refractive error), it seems that for
an overcorrection of the spectacle lens (negative contact lens) solution 4
gives less anisophoria, while with an undercorrection solution 3
gives less anisophoria. To get a subjective experience
of how much (and if) this solution would approximately improve the vision
comfort, the patient should be fitted with a trial frame with standard
trial lenses and a trial contact lens. Alternatively, the patient
could also look through his/her habitual (standard) glasses together with a
size lens, although the amount of induced anisophoria might be a bit
under- or over-estimated in that case (most likely under-estimated).

In the case of field dependent aniseikonia (associated
with retinally-induced aniseikonia), the aniseikonia cannot be fully
corrected by optical means, because an optical correction is approximately
field independent. Nevertheless, a partial correction
often improves the vision comfort considerably. For more
information on field dependent aniseikonia, see our article:
Retinally-induced
aniseikonia1.

If an optical correction is not possible or does
not provide enough correction, a partial occlusion may still be
tried. Some (field dependent) aniseikonia patients have gained some
relief by occluding part of the visual field (for example, placing a (removable)
dot in the center of one of the spectacle lenses or (partially) shielding the
periphery in one eye, creating kind of a tunnel vision for one eye).
Instead of occluding part of the visual field, another approach could be
to occlude the whole visual field, but with a partial transparent foil
(see an article by Silverberg et al.17).

Glossary

Anisometropes: Patients
with anisometropia, i.e. a difference in refractive power
(prescription) between the two eyes. Fig. 1 shows that
anisometropia may induce aniseikonia. The prevalence of anisometropia
is 5-10% of the population above the age of twenty.18

Epiretinal membrane:
Also known as preretinal membrane, cellophane maculopathy, or macular
pucker. A thin membrane growing on the retina that contracts the
retina. Besides a (slight) blurring of the vision, the
contraction may result in (field-dependent) macropsia, i.e. an
enlargement of the perceived image. This monocular macropsia
means that binocularly (field-dependent) aniseikonia will be present.19-22

Macular edema:
Swelling of the retina in the area of the macula (part of the retina
with best visual acuity). May be associated with conditions such as
diabetic retinopathy. May also give rise to micropsia23-25
and therefore aniseikonia.

Macular hole: A
hole in the macula (part of the retina with best visual acuity).
May also give rise to micropsia and therefore aniseikonia26.

Pseudophakes:
Patients with pseudophakia, i.e. the presence of an intraocular lens (IOL)
implant inside an eye, usually after cataract extraction. A
scientific study27 found that 40% of all pseudophakes have ophthalmic complaints referable to aniseikonia. With more than
1.5 million cataract operations per year in the US alone,28 McCormack et
al. stated that aniseikonia can be considered a significant health issue.29

Refractive
surgery: Surgery that corrects visual acuity, with the
objective of reducing or eliminating the need for glasses and contact
lenses. Refractive surgery includes radial keratotomy, PRK, LASIK, and
corneal implants. With the high incidence of aniseikonia in
pseudophakes, an
important question to be answered by researchers is what the incidence of aniseikonia is
in the fast growing group of corneal refractive surgery patients.

Retinal detachment:
Separation of part of the retina from its supporting tissues, caused
for example by aging, trauma, inflammation, high myopia and diseases
such as diabetic retinopathy, and scleritis. After (a prompt)
surgical correction, the visual acuity may be good again, but field
dependent aniseikonia may have developed, with the affected eye often perceiving a smaller image (micropsia).24.30,31

Retinoschisis:
Splitting of the retina. We found that this may give rise to a
(field dependent) macropsia in one direction and micropsia in another
direction.1